Continuity mistake: After Eric pulls the doll from out of the grill, he closes the lid. The lid then alternates between being open and closed throughout the rest of the scene. (00:03:44 - 00:06:15)
Other mistake: Eric is showing Feeny the doll he has and pulls the string to make it talk. When he makes the doll do mission impossible, he pulls the string but the doll doesn't say anything. (00:03:47)
Chosen answer: In the present day, with the United States abiding by more stricter school health laws, for the most part this wouldn't be in school cafeterias as many cannot even have carbonated beverage machines now. However, in the 90s when the show takes place, it wasn't unheard of for there to be coffee machines in the cafeteria of small schools where the teachers eat with the students as we see them do often in the show. The idea being that the cafeteria was small enough so a teacher or hall monitor could catch a student before they could drink the coffee.